This invention relates to a speed change gear for use in excavator and loader vehicles such as shovel bulldozers and tyre shovels etc.
In general, the operation of excavator and loader vehicles is broadly classified into the running operation when all the power generated by the prime mover of the vehicle is used for the power required for running, the running and excavating operation when part of the power generated by the power source is used for running and the rest of it is used for driving implement associated therewith for effecting excavation, and the excavating operation when all the power generated by the prime mover is used for driving the implement.
The general characteristic of a power source such as an internal combustion engine is shown by line "A" in FIG. 1 which shows the relationship between the rotational speed of the engine and the torque generated thereby, while line "B" in FIG. 1 shows the input torque characteristic of a torque converter when an excavating machine is in operation wherein the torque consumed by the implement is deducted from the torque generated by the internal combustion engine.
Further, as regards general specific feature of a torque converter, the torque which can be absorbed from a power source by the torque converter is proportional to the square of the rotational speed of the power source. The line "a" in FIG. 1 shows the torque absorption characteristic of a torque converter having a large torque absorption capability, whilst line "b" shows the torque absorption characteristic of a torque converter having a small torque absorption capability.
In the above-mentioned running operation the torque consumed by the implement is zero, and so the torque generated by the power source (shown by the line "A") becomes equal to the torque absorbed by the torque converter. Therefore, when the torque converter having a large torque absorption capability shown by the line "A" is used, the power used for running can be increased as shown by "T.sub.1 " in FIG. 1.
However, if the implement is used under such condition, the input torque of the torque converter becomes as shown by the line "B" in FIG. 1 so that the relationship of engagement of the power source with the torque converter will change from the state I to II. As a result, the rotational speed of the power source will decrease from N.sub.1 to N.sub.2 and the rotational speed of the pump for driving the implement is also reduced so that the working speed of the implement is reduced.
If in such case a torque converter having a small torque absorption capability as shown by the line "B" is used, the relationship of engagement of the power source with the torque converter will change from the state I to III so that the rotational speed of the power source will change from N.sub.1 to N.sub.3 with little reduction, and as a result, the working speed of the implement is not reduced.
For this reason, the working efficiency of the excavator and loader vehicle can be improved by using a torque converter which can provide a large torque absorption capability as shown by the line "A" when the vehicle, is running and develop a small torque absorption capability as shown by the line "B" when the vehicle is running and excavating.
U.S. Pat. No. 3,478,621 to L. E. Johnson et al. is directed to provide such technology wherein either one of the stator and impeller members of a torque converter is divided into two elements so that one element may rotate relative to the other. A control system comprising signal means may be used with the torque converter for generating a variable signal to control the rotation of one element relative to the other.
Thus, the power absorption capability of the torque converter may be accurately controlled within a predetermined range in response to the level of pressure generated by such signal means.